Improve windeployqt documentation

Make it a clearer that the tool is intended for the Qt-related
dependencies and mention compiler runtime.

Task-number: QTBUG-56566
Change-Id: Idfa938943c486fc5632510bfe9eba119cbcfc58e
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>